The present invention relates to xerographic devices.
In xerographic devices the steps of charging, imaging, developing, transferring and cleaning are well known and are practiced by a variety of means. For example, corona charging, followed by image-reflection discharging, followed by magnetic brush developing, followed by corona transfer, followed by preclean corona charging, and then brush cleaning have been practiced.
By way of example, it is conventional to charge a drum-shaped photoconductor negative, and to thereafter substantially discharge a working portion of the charged photoconductor in all white background areas, to leave a negative latent electrostatic image to be reproduced as a black image on a copy paper. An area of the photoconductor larger than, but including, this working area is then subjected to magnetic brush development. During development, negatively charged and magnetically permeable carrier beads are magnetically transported to a development zone which includes the photoconductor's working area and a nonworking area which borders or surrounds this working area.
The carrier beads are intended to be held within the developer by its magnetic field pattern and by seals which extend between the developer and the photoconductor.
The carrier beads carry small positively-charged toner particles to the photoconductor. As the photoconductor leaves this development zone, the photoconductor's negative latent image is heavily covered with black, positively charged toner, and the remainder of the photoconductor may carry a much smaller amount of randomly distributed toner particles.
Later, the photoconductor, including its now-toned latent image, passes through a transfer zone whereat a piece of white copy paper is placed on the photoconductor, coincident with its working area. The side of this copy paper opposite the photoconductor is subjected to the influence of a negative transfer corona. The paper's negative charge causes a toner image to be formed on the paper, leaving a residual toner image on the photoconductor.
Conventionally, the negative transfer corona spans a width which is greater than the paper, and therefore directly subjects border portions of the photoconductor, and toner particles which may reside thereon, to a negative charge.
The photoconductor, its residual toner image, and its randomly distributed toner particles are now subjected to the influence of a preclean positive corona. This corona tends to neutralize the photoconductor's negative charge, and tends to insure that all particles carried by the photoconductor, including toner, are charged positive. Such particles are charged positive for the purpose of obtaining proper cleaning action by the next xerographic station, namely, the cleaning station. At the cleaning station, the photoconductor is brushed to ideally remove all particles, including toner, from the photoconductor. Conventionally, the cleaning station is constructed and arranged to have an affinity for positively charged particles. It is not unusual, as a part of the cleaning process, to flood the photoconductor with light, to produce the effect of additionally neutralizing any charge carried by the photoconductor.
The prior art, for example U.S. Pat. No. 2,832,977, recognizes the advantage of providing a preclean corona. As a specific improvement, U.S. Pat. No. 4,133,610 provides a specified corona current to aid in the removal of tetrafluoroethylene carrier-coating particles from the photoconductor.
In the well known xerographic device, the above-described process is repeated, once for each copy.
Conventially, xerographic process stations such as the various charging stations and the developer are centered on the photoconductor, and their positions are accurately controlled, both as to centering and as to spacing from the photoconductor. If these parameters are not accurately controlled, problems such as toner filming and bead carryout can occur.
By way of definition, toner filming is that propensity of the toner to gradually coat the photoconductor over a period of many copy cycles so as to produce streaks in the resulting copies. Toner filming usually begins at the outside edges of the photoconductor and "grows" toward its center.
Bead carryout is that propensity of the developer's carrier beads to be carried out of the developer as the photoconductor passes through the development zone.
The prior art suggests solutions to the bead carryout problem. U.S. Pat. No. 3,834,804 provides a magnet which cleans the photoconductor of carrier beads which have been carried out of the developer; whereas, U.S. Pat. No. 3,982,830 alters a developer's development electrode bias voltage as a drum photoconductor's drum seal moves through the developer, to thereby reduce the number of carrier beads which may be carried out of the developer by the drum seal.